In the failure analysis of electronic structures, a specimen of the electronic structure that contains a defect is frequently prepared for microscopic examination. For instance, a specimen of an electronic structure such as a semiconductor wafer is frequently analyzed in a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to study a specific characteristic feature in the semiconductor device. The characteristic feature may include a circuit fabricated or a defect formed during the fabrication process. SEM is one of the most useful equipment for analyzing the microscopic structure of semiconductor devices which provides the benefits of a three-dimensional image for analyzing a structure and a simple specimen preparation method.
In preparing specimens of a semiconductor wafer for electron microscopic examination, various polishing and milling processes are used to section the structure until a specific characteristic feature is exposed. As device dimensions are continuously reduced to the sub-half micron level, the techniques for preparing specimens for study in an electron microscope have become more important. The conventional methods for studying structures by an optical microscope cannot be used to study features in a modem electronic structure due to its low resolution.
Generally, when a specimen is prepared for an electronic microscopic study of a characteristic features, various mechanical polishing and grinding techniques are used to first bring the dimension of a specimen down to a size just before the characteristic feature is reviewed. A final sample preparation step is then accomplished by a method such as ion milling. The ion milling method is frequently conducted by a focused ion beam (FIB) technique.
In a FIB technique, focused ion beams are used to either locally deposit or remove materials. A cluster of ionized beam consists of an aggregate of from 100 to 2,000 atoms is aimed at a sample surface. When the cluster impacts the surface of an electronic structure, the cluster disintegrates into atoms which are then scattered over the surface to remove a surface layer of the material. Typical ion beams have a focused spot size of smaller than 100 nm when produced by a high intensity source. Sources of such high intensity ions can be either liquid metal ion sources or gas field ion sources. Both of these sources have a needle type form that relies on field ionization or evaporation to produce the ion beam. After the ion beam is produced, it is deflected in a vacuum and directed to a desired surface area. The focused ion beams can be suitably used in semiconductor processing industry as a cutting or attaching tool to perform a circuit repair, a mask repair or a micro-machining process.
A cutting or milling process is normally performed by locally sputtering a surface with a focused ion beam. In an ion beam milling process, where a material is selectively etched by a beam of ions such as Ga.sup.+ focused to a sub-micron diameter, the technique is often referred to as focused ion beam etching or milling. FIB milling is a very useful technique for restructuring a pattern on a mask or an integrated circuit, and for diagnostic cross-sectioning of micro structures. In a typical FIB etching process, a beam of ions such as Ga.sup.+ is incident onto a surface to be etched and the beam can be deflected to produce a desirable pattern. In the etch chamber, a gas such as Cl.sub.2 can be introduced to fill the chamber to a pressure of about 30 m Torr, while the vacuum outside the chamber where the FIB is generated is normally maintained at approximately 10.sup.-7 Torr. The focused ion beam can be used to bombard a specimen surface at a very low angle, i.e., as low as 0.about.5.degree., such that a cavity can be formed on the surface of an electronic structure to reveal a characteristic feature of the structure for electron microscopic examination.
In the preparation of a sample surface by the FIB technique, extreme care must be exercised to reveal the characteristic feature without damaging it such that an accurate examination can be made. A FIB technique is most suitable for preparing such a microscopic sample when a cavity is cut into the sample since the technique can be used to simultaneously cutting and observing the sample surface at the same time. Furthermore, the sample specimen in a FIB machine can be tilted to any desirable angle less than 50.degree. such that a cavity of a desirable shape can be formed in the surface. The FIB technique utilizes a primary beam of ions for removing a layer of material at a high current, and then observing the surface that was newly formed at a low current. The observation of the surface is made by detecting the secondary electrons emitted from the sample surface when the surface is bombarded by the ions. A detector is used to receive the secondary electronics emitted from the surface to form an image. The FIB method, even though cannot produce an image at a high resolution such as that obtainable in a SEM/TEM technique, can be used to sufficiently identify a newly formed cross-sectional surface which may contain the characteristic feature to be examined. The capability of the FIB technique for making observations down to a resolution of 5.about.10 nm enables the cutting of a precise plane in an electronic structure such that it may be later examined by a SEM or TEM technique at a higher resolution than that available from FIB.
Parallel lapping is a technique for preparing a planar specimen which shows the plane, instead of a cross-section, of an electronic structure. Conventionally, a polishing or grinding technique is used to gradually removing layers from an outer surface toward the interior of the specimen until the specific layer containing a characteristic structure is reached. The structure is then etched in a wet etchant such that it may be observed in SEM for studying the characteristic feature or defect and its reason for being defective. Several problems are encountered when the parallel lapping technique is used to prepare a planar specimen. First, when the parallel lapping technique is used to remove layers in a parallel plane of the specimen, the peripheral area of the specimen is normally removed at a higher speed while the center of the specimen is removed at a lower speed. As a result, a non-planar surface is normally obtained by the parallel lapping technique. Secondly, an optical microscope is normally used to examine the surface layer obtained after the parallel lapping process. However, many characteristic features to be observed, i.e., a failed bit in a memory device, cannot be readily observed under an optical microscope, since the maximum magnification ratio obtainable in an optical microscope is only about 2000.times.. It is therefore difficult to identify a minute structural feature under the optical microscope and to determine whether a suitable layer has been removed and revealed in the specimen.
Attempts have been made in using SEM to determine the polishing layers, or whether a characteristic feature has been revealed by the specific layer. The SEM technique requires an elaborate specimen preparation technique and the attainment of a high vacuum in the specimen chamber which is time consuming and requires a high level of skill.
The FIB technique is therefore a suitable method for sample preparation for examination by SEM or TEM. A typical focused ion beam arrangement is shown in FIG. 1. The FIB apparatus 10 is constructed of a suppressor and a liquid metal ion source 12 wherein the liquid metal ion may be gallium, an ion extractor 14, a three-element asymmetric lens 16, and an electrostatic aperture adjustment 18. The ion beam 20, after crossing over at the aperture 18, is treated by a second three-element asymmetric lens 24 before the ion beam 28 is impinged on a target 30. The operating modes include a beam control pattern which intermittently blanks the ion beam and allows the ion charge to dissipate, thus reducing electrostatic discharge damage. The ion beam 20 is advantageously focused with a column that includes the three-element asymmetric lens assembly 16 and 24. The ion beam 28 produced by the apparatus 10 has superior position stability and high current density. The ion beam further has a fine probe diameter and beam placement accuracy capable of high performance versatility of a full digital beam scanning process.
It is therefore an object of the present invention to provide a method for preparing a small area parallel lapping specimen by a focused ion beam technique which does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique by utilizing a multi-stage ion beam etching process.
It is a further object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique in which an ion beam of a high beam current is first used to remove a layer of the specimen for exposing a surface immediately adjacent to a characteristic feature to be examined.
It is another further object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique in which a high beam current is first used to remove a surface layer of the specimen and then followed by a low beam current for defining a small window area which contains a characteristic feature to be examined.
It is still another object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique in which a high beam current is first used to remove a surface area and define a small window area, followed by a low beam current for removing a second surface area in the small window section to expose a characteristic feature.
It is yet another object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique in which a high beam current of at least 400 pA and a low beam current of less than 200 pA are utilized in different stages of the specimen preparation process.
It is still another further object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique in which a wet etching process is conducted after the ion beam milling process to prepare a three dimensional structure of the characteristic feature for microscopic examination.
It is yet another further object of the present invention to provide a method for preparing small area parallel lapping specimens by a focused ion beam technique in which a high beam current and a low beam current are both used with the low beam current not more than one half of the high beam current.